The present invention relates to a method and apparatus for the production of prereduced iron ore, Direct Reduced Iron (DRI), sponge iron, or the like by the gaseous reduction of iron-oxide containing particles (iron oxide lump ore), in a reduction system which comprises a preheating device for said particles, a reduction reactor, a source of natural gas, and a heating device. The natural gas is transformed in the operation of said system by means of a reformer or by the catalytic action of the metallic iron within said reduction reactor, to a reducing gas having carbon monoxide and hydrogen as its main constituents. The heating device heats said reducing gas prior to its introduction into said reduction reactor. More particularly, the invention relates to a method and apparatus which preserves the strength of iron-oxide-containing particles while they are reduced to metallic iron. The pre-reduction strengthening allows for the use of mechanically weak iron-oxide-containing particles as the charge for said reduction reactor while avoiding the sticking and fines problems associated with the use of said particles. The invention provides a method and apparatus for processing iron ore particles which otherwise would not be possible in a moving bed direct reduction process.
The invention also provides other advantages as, for example, that the reducing gas reaches a high degree of oxidation in the reduction reactor whereby the productivity of the reduction system increases as a result of the decreased amount of reducing gas as well as the decreased residence time required. A further advantage produced by the invention is that the recycled reducing gas can be preheated by heat-exchange with the gas stream effluent from the reduction reactor at a higher temperature as a result of the preheating of the iron ore particles.
In steelmaking processes and specifically in direct reduction processes, it is well known to those skilled in the art that the structural changes and chemical reactions that take place in the iron-oxide-containing materials, cause mechanical weakness and disintegration, especially when said materials are in the form of lumps and/or pellets.
During the chemical transformation of the ore particles comprising iron oxides such as hematite and/or magnetite to metallic iron, there are several steps and methods to be followed. The present invention relates to a method and apparatus that provides a successful way to maintain the strength of the iron ore lumps or pellets throughout the reduction process.
Without committing the patentability and functionality of the present invention to a particular theory, and based on currently available technical literature, the applicants propose that the structural changes caused by the removal of oxygen from the iron-oxide material, represents one cause of the weakening of the iron ore lumps and pellets. The reduction reaction from Fe2O3 to Fe3O4 (hematite to magnetite respectively) changes the crystalline structure of the iron-oxide containing particles. Yaginawa et al. (xe2x80x9cTheoretical and experimental study on the reduction of iron ore pellets in moving bedxe2x80x9d, Proceedings of the Third International Iron and Steel Congress, Apr. 16-20, 1978, published by American Society for Metals, pages 449-459.) found that the decrease of strength was slight if the reduction time was under 75 minutes. In the prior art, the continuous feeding of iron-oxide containing particles is carried out at ambient temperature, it takes a significant time to heat and reduce the material (about 3 hours). The present invention comprises the pre-heating of the material in an oxidizing atmosphere so as to preserve the strength of the crystalline structure. Preheating the iron-oxide containing particles to 700xc2x0 C. in the reduction reactor, allows the retention time between the hematite to magnetite to be decreased, and allows a smooth change between the stereometry between the structures of hematite and magnetite.
Another cause for the weakening of the iron-oxide containing particles is believed to be that during heating and reduction of iron-oxide materials from an ambient temperature to a high temperature (above 650xc2x0 C.), the strength of the iron-oxide containing particles decreases significantly in the temperature range between 700xc2x0 to 1,100xc2x0 C. After this range, strength is recovered by the formation of the metallic iron in the material structure.
Osman (U.S. Pat. No. 3,684,486) describes a reduction process where the iron ore is heated in a fixed bed reactor above said temperature range using an oxidizing gas and the iron ore is subsequently reduced. Even though Osman detected the effect of preheating the iron ore before reduction on the strength of the sponge iron produced, this reference does not suggest or disclose the great and surprising benefits thereof when the ore is reduced in a moving bed reactor where the particles are subjected to inter-particle movements and pressures and therefore are much more prone to disintegration and fines production. A person skilled in the art of moving bed reduction systems hardly would look for a solution to these problems in a reference disclosing a reduction system in a fixed bed, because these problems are different conceptually and in practice. The main difference between the reduction process in a moving bed and the reduction in a fixed bed, is that the reaction front in a fixed bed is moving along the reactor, and when the reduction is performed in a moving bed, the reaction front is stationed at a specific level of the reactor.
The present invention comprises a step of preheating iron ore with an oxidizing gas applied to a moving bed reactor, thus avoiding the formation of fines within the temperature range between 750xc2x0 and 1,100xc2x0 C. caused by the low strength, abrasion, ferrostatic pressure and motion. Said weakening in the structural strength of the iron ore in continuous reactors causes the formation of fines, which in turn produces several problems in the performance of the reactor operations such as hot spots, channelization, agglomerations, etc.
Another advantage of preheating the iron-oxide-containing particles and of feeding in a continuous mode said iron-oxide-containing particles to the reactor at high temperature, is that the requirement of a reducing gas is diminished. Since the heating potential of the reducing gas is not required to heat the charge from ambient temperature to at least 700xc2x0 C., those skilled in the art can visualize the advantageous use of the reducing gas.
This invention is herein described as applied to lumps of iron ore, but it will be evident to those skilled in the art that it is also applicable to pellets, sinter or otherwise agglomerated iron oxides.
It is an object of the present invention to provide a method and an apparatus for producing DRI in a reduction system which avoids the problems caused by the decreased strength of iron-oxide-containing particles inside the reactor such as agglomeration, channelization and sticking.
It is a further object of the invention to provide a method and an apparatus for reducing the overall energy requirements or increasing the plant capacity at the same rate of energy consumption.
According to the present invention, the objects thereof are achieved by providing a method and apparatus by the following preferred embodiment:
A method for producing DRI from iron ore particles, in a reduction system whereby a preheating device heats said particles to a temperature above 700xc2x0 C. in an oxidizing atmosphere before the particles enter the reduction reactor, thus avoiding the formation of fines inside the reactor shaft.
Said reduction system comprises a moving bed reduction reactor having a reduction zone where iron-oxide-containing particulate materials are, at least partially, chemically reduced to metallic iron by a high-temperature reducing gas which comprises hydrogen and carbon monoxide as reducing agents. Said reduction reactor also comprises a discharge zone where said metallic iron lumps are discharged in either a hot or cold mode.
Said method comprises introducing the iron-oxide-containing particles into a preheating device where said particles are contacted with a hot non-reducing gas stream; heating said particles to a temperature in the range of 75xc2x0 C. to 1,100xc2x0 C. discharging said hot iron-oxide-containing particles into the reduction zone of the reduction reactor at a temperature above 700xc2x0 C.; contacting said particles with the reducing gas reducing said iron-oxides to metallic iron; withdrawing said sponge iron particles through the discharge zone; and withdrawing off-gas from said reduction zone at a temperature above about 500xc2x0 C. and exchanging heat from said hot off-gas with another gas stream of the reduction system; regenerating the reducing potential of at least a portion of said off-gas by removing at least one of the oxidants produced in the reduction reactor H2O and CO2 therefrom and optionally, utilizing said regenerated off-gas along with natural gas to produce more reducing gases in a reformer or in said reduction reactor.